The Coggins test is a blood test for antibodies for Equine Infectious Anemia. EIA positive horses are required to live in perminent quarentine or destroyed (euthanized) to prevent the disease from spreading. The argument people use against the test, claiming its the product of money hungry vets, is that the test is only valid for the horse BEFORE the blood is drawn. After the blood is drawn, the horse could get infected. So theoretically a horse could get infected 10 minutes after being tested, and spread the disease everywhere for an entire year until the test "expires". Hence why I do my testing twice a year, before mesquitos come out, and after they die.
I too have known horses that were destroyed because they tested positive (they do perform multiple tests before condeming a horse to die), and I rather my horse be euthanized before they spread it to too many others, hence why I pay for more strict testing.
It is federal law to have a negative coggins test preformed less than 12 months before crossing any state boarder. Some states have stricter laws (most, in fact) requiring proof of ownership and/or 6-12 month negative coggins test to be on any public road. Other states do not require Coggins for instate movement (Colorado did not require instate coggins, but did require Brand Inspections... Kentucky requires coggins and health certificate for any movement, intrastate or interstate.)